In order for a print imaging support to be widely accepted by the consumer for imaging applications, it has to meet requirements for preferred basis weight, caliper, stiffness, smoothness, gloss, whiteness, and opacity. Supports with properties outside the typical range for ‘imaging media’ suffer low consumer acceptance.
Traditional photographic prints, as well as ink jet, thermal and all other reflective imaging methods need to have a smooth surface in order to provide the image viewer with a visually pleasing surface that provides uniform gloss and reflective properties. When prints have a high level of roughness, light will reflect off the surface at different angles in relation to the viewer and therefore present an unappealing image. Such a rough surface may also result in nonuniform exposure of photographic images and result in images that are not sharp.
Another consideration is the opacity of a reflective support. Adequate opacity prevents the show through of the reflective image below the one being viewed in a stack of images or the non white surface that the reflective image is resting on or to which the image is mounted. Given the fact that ink jet, thermal, and most high end imaging media were derived from and are now in competition with photographic imaging media, the need for comparable degrees of opacity becomes necessary.
In addition to these fundamental requirements, imaging supports are also subject to other specific requirements, depending upon the mode of image formation on the support. For example, in the formation of photographic paper, it is important that the photographic paper be resistant to penetration by liquid processing chemicals. In the formation of ‘photo-quality’ ink jet paper, it is important that the paper is readily wetted by ink and that it exhibits the ability to absorb high concentrations of ink and dry quickly. For thermal media, it is important that the support contain an insulative layer in order to maximize the transfer of dye from the donor that results in higher color saturation.
Additionally, there is a need for imaging prints that are light in weight and that are sufficiently stiff to provide the consumer with not only a visually appealing print but also one that feels substantial and not flimsy. High stiffness is also desirable in order to resist curl. This is particularly important for photographic prints in which the gelatin of the photographic emulsion develops a very high modulus in low relative humidity conditions that cause the print to curl towards the image layer.
It is important, therefore, for an imaging media to simultaneously satisfy several requirements. One commonly used technique in the art for simultaneously satisfying multiple requirements is through the use of composite structures comprising multiple layers wherein each of the layers, either individually or synergistically, serves distinct functions. Multiple operations are typically required to manufacture and assemble all of the individual layers into a single support. For example, it is known that a conventional photographic paper comprises a cellulose paper base that has applied thereto layers of polyolefin resin, typically polyethylene, on each side, which serve to provide waterproofing to the paper and also provide a smooth surface on which the photosensitive layers are formed. U.S. Pat. No. 5,866,282, discloses biaxially oriented polyolefin sheets extrusion laminated to cellulose paper to create a support for silver halide imaging layers. The composite imaging support structure described has been found to be more durable, sharper, and brighter than prior art photographic paper imaging supports that use cast melt extruded polyethylene layers coated on cellulose paper. In U.S. Pat. No. 5,851,651, porous coatings comprising inorganic pigments and anionic, organic binders are blade coated to cellulose paper to create ‘photo-quality’ ink jet paper. Photographic paper typically requires a paper making operation followed by a polyethylene extrusion coating operation, or as disclosed in U.S. Pat. No. 5,866,282, a paper making operation is followed by a lamination operation for which the laminates are made in yet another extrusion casting operation. There is a need for imaging supports that can be manufactured in a single in-line manufacturing process while still meeting the stringent features and quality requirements of imaging bases.
It is also well known in the art that traditional imaging bases consist of raw paper base. Although raw paper base is typically a high modulus, low cost material, there exist significant environmental issues with the paper manufacturing process. There is a need for alternate raw materials and manufacturing processes that are more environmentally friendly and minimize environmental impact, without sacrificing the imaging base features that are valued by the customer, that is, strength, stiffness, and surface properties of the imaging support. In addition, existing composite paper structures are typically subject to curl through the manufacturing, finishing, and processing operations, leaving a need for an imaging support that minimizes curl sensitivity as a function of humidity, or ideally, does not exhibit curl sensitivity.
Recently, attempts have been made to replace paper support the use of closed cell foam core imaging elements, as described in U.S. Pat. Nos. 6,447,976, 6,514,659, 6,537,656, 6,566,033 and U.S. Patent Application 2003/0152760A1. The ‘polymer foams’ have previously found significant application in food and drink containers, packaging, furniture, and appliances. Polymer foams have also been referred to as cellular polymers, foamed plastic, or expanded plastic. Polymer foams are multiple phase systems comprising a solid polymer matrix that is continuous and a gas phase. For example, U.S. Pat. No. 4,832,775 discloses a composite foam/film structure which comprises a polystyrene foam substrate, oriented polypropylene film applied to at least one major surface of the polystyrene foam substrate, and an acrylic adhesive component securing the polypropylene film to the major surface of the polystyrene foam substrate. The foregoing composite foam/film structure may be shaped by conventional processes as thermoforming to provide numerous types of useful articles including cups, bowls, and plates, as well as cartons and containers that exhibit excellent levels of puncture, flex-crack, grease and abrasion resistance, moisture barrier properties, and resiliency.
Foams have also found limited application in layers in combination with paper or other support for imaging media. For example, JP 2839905 B2 discloses a 3 layer structure comprising a foamed polyolefin layer on the image receiving side, raw paper base, and a polyethylene resin coat on the backside. The foamed resin layer was created by extruding a mixture of 20 weight % titanium dioxide master batch in low density polyethylene, 78 weight % polypropylene, and 2 weight % of Daiblow PE-M20 (AL)NK blowing agent through a T-die. This foamed sheet was then laminated to the paper base using a hot melt adhesive. The disclosure JP 09127648 A highlights a variation of the JP 2839905 B2 structure, in which the resin on the backside of the paper base is foamed, while the image receiving side resin layer is unfoamed. Another variation is a 4 layer structure highlighted in JP 09106038 A, in which the image receiving resin layer comprises 2 layers, an unfoamed resin layer which is in contact with the emulsion, and a foamed resin layer which is adhered to the paper base.
Approaches are known in the art of preparing resin coated paper imaging supports with a specific smoothness. For example, in Application EP 0952483, a paper support for photographic printing having a pigmented coating based on clay and/or other pigment and an average surface roughness RA of 1.0 μm or less, is provided with at least one pigmented polymer resin layer. This application is specific to paper bases that are resin coated but there are limitations resulting from the properties of the paper bases. The smoothness of the base paper is reportedly increased by calendering the paper at high pressures between metallic rolls. Limitations of this method are that calendering will reduce the thickness of the base paper and result in a decrease of whiteness and stiffness. In addition, this high pressure calendering method has been attempted to suppress the crater defects by increasing the thickness of the polymer resin coating layer. At high extrusion speeds, such as over 300 m/min this is not sufficiently effective. With higher coverage, features with longer wavelengths are filled or leveled more. However, at some point, it is not economical to apply higher and higher resin coverage, as the increase in polymer resin thickness increases production costs of photographic printing paper. JP-B 06-048365 discloses coating the base paper sheet followed by gloss calendering prior to the melt extrusion coating of titanium oxide filled polymer resin. According to this patent, the gloss of the photographic paper is related to the gloss of the base paper. This latter value is improved by providing a pigmented surface coating on the base paper followed by a gloss super-calender treatment, prior to laminating with the titanium oxide filled polymer resin. The required high gloss value was achievable by providing more than 50 wt. % of kaolin in the pigmented surface coating.
Unlike paper supports, foam support is typically so rough that very high coverage of the extrusion polymer is required to provide the necessary smoothness desired by the consumers. Commercially available foam cores are several times rougher than conventional imaging paper bases. Typically paper bases for imaging support have a roughness of from 0.6 to 1.4 microns while foams are typically at least from 2 to 4.5 microns. Conventional resin coated paper has a resin coverage of from 24 to 29 g/m2 to produce the desired roughness of the final image but conventional foam cores need to be coated with 58 to 75 g/m2 to achieve acceptable roughness. Foam supports also suffer from surface pits and craters as a result of the foaming process. An alternative approach for smoothing closed cell foam core is needed as a result of the inherent roughness of the closed cell foam core.